1. Field of the Invention
The present invention relates to a directional coupler using a dielectric guide, and to an antenna device and a transceiver using the directional coupler.
2. Description of the Related Art
Conventionally, a dielectric guide, comprising a dielectric strip provided between two conductive plates, is used as a guide for transmission in the milliwave band and the like. When forming a milliwave circuit using such dielectric guides, a directional coupler is used in a portion where electrical power is split between two dielectric guides.
A conventional directional coupler using dielectric guides comprises two dielectric strips, having a linear portion and a curved portion, which are provided at a predetermined distance apart between two conductive plates. The dielectric strips are arranged close together and the dielectric guides are coupled at this closely arranged portion.
A milliwave radar is an example of the milliwave circuit using dielectric guides described above. An antenna device used in a milliwave radar comprises a dielectric lens and a primary emitter provided at the focal point of the dielectric lens.
However, since the direction of the antenna in the conventional milliwave radar is fixed, in certain conditions it is not possible to achieve the intended sensitivity and measurements, as will be explained below. For instance, as shown in FIG. 22, when a vehicle is traveling on a multi-lane road, it is not possible to determine immediately whether other vehicles in front of it are traveling in the same lane based only on waves reflected from the other vehicles. That is, in FIG. 22, when the vehicle Cm emits a beam B2, it picks up not only waves reflected from the vehicle Ca which is traveling in front, but also waves reflected from the vehicle Cb which is traveling in the opposite lane. Furthermore, as shown in the example of FIG. 23, when the vehicle Cm emits a beam B1 in the forward direction, it is unable to detect the vehicle Ca which is traveling in front in the same lane. Moreover, as shown in FIG. 24, when traveling on an curving road, even though the vehicle Cm emits a beam B1 in the forward direction, it cannot detect the vehicle Ca in front of it.
One conceivable solution is to provide an antenna device, combining a primary emitter and a dielectric lens, in which the direction of the beam is tilted by changing the position of the primary emitter. In order to change the position of the primary emitter, the configuration should be arranged so that the dielectric guide connecting to the primary emitter and the other dielectric guide connecting to the circuit can be relatively displaced while remaining coupled with low loss. To achieve this, the configuration of the directional coupler using dielectric guides described above need only be arranged so that two dielectric guides can be relatively displaced.
However, the separated positions (separated faces) of the two dielectric guides of the directional coupler are parallel to the two dielectric strips provided closely together. With this configuration, the end faces of the conductive plates on either side of the dielectric strips are provided parallel to the direction of propagation of the electromagnetic waves of the two dielectric guides, and consequently the path of the current flowing through the conductive plates is broken at the end face portions of the conductive plates, causing reflection. As a result, there are problems such as the creation of unwanted modes other than the propagation mode, increased loss, or an inability to obtain desired characteristics of the directional coupler, etc.
The above example describes case where the two dielectric guides of the directional coupler portion are relatively displaced, but the directional coupler can be used when forming a single device incorporating circuit modules using dielectric guides, to couple the dielectric guides between the circuit modules. In this case too, the path of the current flowing through the conductive plates is broken between the circuit modules, causing reflection. As a result, there are problems of increased loss and inability to obtain desired signal transmission characteristics between the circuit modules.